Apparatus for measuring and controlling mixture content

ABSTRACT

Apparatus for detecting the ratio of component particles in a mixture wherein the quantity of one of the component particles is continually decreased, as in an electrostatic printing operation, includes the provision of a window adjacent the path of movement of the mix of component particles and contacting particles in transit therepast, illumination of the mix through the window and detection of light reflected from the mix through the window. Replenishment of the depleted component may be electrically controlled in accordance with the light reflected and optical filter provisions may be employed to limit the spectrum of the light with which the mix is illuminated where the components of the mix differ in color. Diversion of a quantity of mix to bypass an area wherein depletion of one component occurs effects mixing in the direction of flow and crossflow mixing effects greater consistency in the mix transverse to the direction of flow.

United States Patent 21 Appl. No. 768,377

[22] Filed Oct. 17, 1968 [45] Patented Oct. 5, 1971 [73] AssigneeContinental Can Company, Inc.

New York, N.Y.

[54] APPARATUS FOR MEASURING AND CONTROLLING MIXTURE CONTENT 14 Claims,6 Drawing Figs.

[52] US. Cl 118/637, 101/1 14, 1 17/17.5, 1 18/9, 118/603, 222/57,356/201 [51] Int. Cl B05bll/00, B05c 5/02 [50] Field otSearch 118/637.7,603; 117/17.8, 37; 101/1 14; 222/57; 356/201 [56] References CitedUNITED STATES PATENTS 2,956,487 10/1960 Giaimo,Jr. 1l7/17.5 X 3,094,0496/1963 Snelling 118/637 X 3,233,781 2/1966 Grubbs 118/637 UX 3,306,1932/1967 Rarey et al. 118/637 X 3,336,905 8/1967 Lehmann 188/637 3,376,8534/1968 Weiler et al.. 1l7/l7.5 X 3,399,652 9/1968 Gawron l17/l7.5 X3,430,606 3/1969 Pease et al. 117/1 7.5 X

Primary Examiner-Peter Feldman Attorney-Diner, Brown, Ramik & HoltABSTRACT: Apparatus for detecting the ratio of component particles in amixture wherein the quantity of one of the component particles iscontinually decreased, as in an electrostatic printing operation,includes the provision of a window adjacent the path of movement of themix of component particles and contacting particles in transittherepast, illumination of the mix through the window and detection oflight reflected from the mix through the window. Replenishment of thedepleted component may be electrically controlled in accordance with thelight reflected and optical filter provisions may be employed to limitthe spectrum of the light with which the mix is illuminated where thecomponents of the mix differ in color. Diversion of a quantity of mix tobypass an area wherein depletion of one component occurs effects mixingin the direction of flow and crossflow mixing effects greaterconsistency in the mix transverse to the direction of flow.

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will )5 Wm OP'HCQL REFLEEUON DENS\TY 4 PATENTED 0m 519m SHEET 2 OF 2\.'o O/OTONER XEROX 22-\42,BLUE

TONER REMOVE D FROM M\XTURE Q 58 5 AMPUHER mums Ila-6 PREFERRED sT ATONTONER DEHmEuT REumvE comm WXTURE 0F conwouems f 65 A REURCULRTNGM\X\'URE TO nccoMPusH MDUNG N THE D\RECTIDN o; DEVELOPER mom 68woNEggooel) 63 72 M\XTURE MWESME 313? 4 smvou STRHON 14 w 1 4k FEEDBHCKLOOP-S\GNAL FROM opncm. MOWER use!) To comm, TONER INVENTORS ADDNG DEVCEKENNETH \ALRHREY an JOHN B.KENNEDY,Jr.

ATTORNEYS APPARATUS FOR MEASURING AND CONTROLLING MIXTURE CONTENT Thisinvention relates generally to apparatus for measuring and controllingthe ratio of components in a mixture of two or more components and moreparticularly to apparatus for optically detecting and electricallycontrolling the ratio of toner to carrier particles in an electrostaticprinting operation.

Previously optical-monitoring of mixtures to indicate the ratio ofcomponent particles therein has been employed. However, previously knownmethods and apparatus have been marked by several disadvantagesincluding the tendency of the employed optical-monitoring system toexhibit a reduction in sensitivity on prolonged use resultant from thedusting or collection of particles on surfaces of the optical system.Similarly, previously known optical-monitoring system have failed toprovide for differences in color between the component particles of amixture to provide an indication of the ratio of those componentparticles based upon such differences in color. Further, previouslyknown optical-mixture monitoring systems and control provisions havefailed to provide either methods or apparatus for correcting the ratioof component particles in a moving mix while providing a complete mixingof those particles both in the direction of flow and transverse thereto.

In view of the foregoing, it is an object of this invention to provideapparatus for optically monitoring the ration of component particles ina mixture of such particles without a reduction in sensitivity throughthe build up of particles upon portions of the optical system.

In accordance with the immediately foregoing object, it is a furtherobject of this invention to provide an optical-monitoring systemincluding a window adjacent the path of movement of the mix and having asurface over which particles of the mix slide in automatic cleaningaction, and light source and photosensitive provisions for illuminatingthe mix in transit past the window and for detecting light reflectedfrom the mix.

A further object of this invention is to provide apparatus forindicating the ration of component particles in a mix through theemployment of optical-monitoring provisions including a light source forilluminating a portion of the mix, photosensitive means for receivingthe light reflected from the mix and an optical filter for limiting thespectrum of light projected against the mix, causing the degree ofreflected light to depend largely upon the ration of component particlesof differing color.

Still another object of this invention is to provide apparatus forincreasing the consistence of a mixture of two or more componentparticles which are moved to and away from an area wherein the quantityof one of the component particles is decreased and including thediversion of a quantity of the particles to bypass the aforesaid area,providing mixing of the component particles in the direction of flow ofthe mix and providing mixing of the component particles transverse tothe direction of flow of such particles to provide a consistency of mixacross the flow thereof.

Still another object of this invention is to provide in an electrostaticprinting or coating apparatus provisions for optically monitoring acarrier mix of carrier and toner particles, means responsive to theoptical-monitoring of the mix for providing an electrical signalindicative of the ratio of component particles in the mix and means forreplenishing toner in the developer mix upon occurrence of an electricalsignal of a predetermined magnitude.

With these and the further objects in view that will hereinafter appear,the nature of the invention will be more clearly understood withreference to the several figures of the accompanying drawings, thefollowing description and the appended claims.

IN THE DRAWINGS FIG. 1 is a diagrammatic view of electrostatic printingapparatus, and shows the optical provisions for monitoring the employeddeveloper mix, a first mix-transporting drum, a second mix-divertingdrum and crossflow-mixing provisions.

FIG. 2is a graphical illustration potting the variation in opticaldensity of a quantity of developer mix against variations in thepercentage toner particles in the mix.

FIG. 3 is a graphical illustration plotting variation in the opticaldensity of a quantity of developer mix employing red toner particlesagainst variations in the percentage toner particles in the mix andillustrates the variations in optical density resultant from thealternate employment of a blue and a red filter illuminating thedeveloper mix.

FIG. 4 is a graphical representation plotting optical density of aportion of developer mix including blue toner particles againstvariations in the percentage toner particles in the developer mix andshows the variations in optical density resultant from the alternateemployment of a red filter and a blue filter in the illumination of thedeveloper mix.

FIG. 5 is a diagrammatic illustration of the operation of theelectrostatic printing apparatus according to FIG. I and illus tratesthe movement of developer mix in such apparatus, including the diversionof recirculation of a portion of the mix and the crossflow-mixingfollowing toner addition.

FIG. 6 is an enlarged diagrammatic view of the optical system of FIG. Iand illustrates the employment of a photosensitive sensor and meterrelay combination for indicating the ration of component portions of themix and for controlling toner addition.

Referring to the drawings in detail, FIG. 1 illustrates an electrostaticprinting device, generally referred to by the numeral 10, of a typesuitable for use in the practice of this inventionand similar to theelectrostatic printing device shown and described in the US. Pat. to K.W. Rarey and J. B. Kennedy, Jr., No. 3,306,193. The electrostaticprinting device or printing machine 10 includes a suitable housing II,shaped to define a developer mix depository 12. The mix depository I2 ispositioned to receive a quantity of developer particles and tonerparticles which may be commercially available or of a type suitablyselected for employment in the electrostatic printing or coatingprocess.

Disposed adjacent the depository 12 of the printing machine 10 is arotatable magnetic feeding drum I3 which, upon rotation, transports aquantity of the developer mix upwardly into adhering relation with anelongate conductive belt electrode 14. The conductive belt I4 isentrained for movement about a plurality of belt support pulleys l5 andan upper run 16 of the endless conductive belt 14 conveys apredetermined quantity of mix into the printing zone, generally referredto by the numeral 17, of the overall printing apparatus 10. A firstdoctor blade 18 is arranged adjacent the exterior of the cylinder 13,providing a backup of developer mix adjacent the belt electrode 14 and asecond doctor blade 20 is arranged in proximity to the belt electrode14, providing an even distribution of developer mix upon the upper run16 of the belt electrode 14 within the printing zone 17. Theestablishment of an appropriate field or fields in the printing zone 17draws the particles of developer upwardly into contact with a screenstencil 21 and a portion of the toner particles of the developer mix areremoved from the carrier particles for the production of an image orcoating upon a substrate 22, as determined by the configuration of themasking of the screen stencil 2]. It will, of course, be evident thatthe ratio of carrier particles to toner particles is altered within theprinting zone 17 and the developer mix is conveyed via the conductivebelt electrode I4 away from the printing zone 17 for return to thedepository 12.

The quantity of toner particles in the developer mix is replenishedduring movement of the developer mix from the printing zone 17 to thedepository I2 via a surface 23 provided upon an interior portion of thehousing 11. A toner reservoir 24 provides a source for replenishing oftoner in the developer mix from a supply 25 of toner housed therein. Ascreened window 26 communicates through the surface 23 between theinterior of the housing 11 and the reservoir 24 and a rotatable brush 27effects passage of toner particles through the screened window 26 anddeveloper particles en route from the printing zone 17 to the depository12 pass across the screened window 26. The screen mesh of the window 26is sufficiently fine to prevent passage of carrier particlestherethrough while permitting rotation of the brush 27 to effectreplenishment of the toner in the developer mix recently employed in theprinting operation.

The replenished developer mix continues past the screened window 26,along the interior surface 23 of the housing III and into the depositoryll2. Crossflow-mixing provisions generally indicated by the numeral 28are provided immediately following the screened window 26 along the pathof movement of the developer mix and include a plurality of pins 30mounted between the surface 23 and a guide plate 311 lying parallel toand above the surface 23. The pins 30 intersect the path of flow of thedeveloper, providing obstacles with which the developer particlescollide to cause those particles to move at least partially transverseto the general direction of flow of the developer during its passagethrough the crossflow-mixing provisions 28. Accordingly, mixing of thedeveloper in the direction transverse to the flow of the developer mixis effected, providing greater homogeneity of the mix in the directiontransverse to the general flow thereof.

A blade 32 mounted adjacent the outer periphery of the magnetic drum ll3intersects movement of a portion of the developer particles carried uponthe periphery of the drum l3 and a further magnetic drum 33 having itsperiphery adjacent the blade 32 and on the opposite side thereof fromthe firstmentioned magnetic drum 13 carries the portion of developer mixintersected by the blade 32 away from the main magnetic drum I3 and anadditional doctor blade 34 removes the diverted developer mix from thesurface of the drum 33, returning that portion of the mix into thegeneral flow of developer from the printing zone 17. The blade 32 andmagnetic drum 33 effect mixing of the developer in the direction of flowof the developer by diversion of a quantity of the mix and reapplicationof that quantity into the mix flowing along the surface 23 of thehousing 11.

A window 35 communicates between the interior and exterior of thehousing II adjacent the flow path of developer within the overallprinting apparatus I and is placed along that path of flow immediatelyfollowing the cross-mixing provisions 28. The window 35 defines aninterior surface 36, coplanar with the interior surface 23 of thehousing 111 and providing a surface over which developer mix flowsduring its passage past the window 35. Attention may be given to the location of the window 35 to assure that the developer contacting theinterior surface 36 is in motion and in sliding contact with the surface36 to provide automatic cleansing of the interior surface, substantiallyreducing the dusting effect resultant from a build up of toner depositupon the interior surface such as reduces the transparency provided bythe window 35 during operation of the printing apparatus 10.

A light source 37 which may include an associated lens or lens system 38is positioned to project a light beam through the window 35 and againstthe developer mix in transit therepast. A photosensitive detector orsensor 40 is positioned to receive light reflected through the window 35from the mix in transit therepast and a further lens or lens system 41may be provided for focusing the reflected light upon the photosensitivedetector 40. A colored optical filter i2 is shown intersecting the lightwith which the developer mix is illuminated to limit the spectrumthereof and to provide greater sensitivity of the photosensitivedetector 410 to variations in the ratio of toner to carrier, as will bediscussed more fully hereinafter.

In an electrostatic printing operation such as that provided by theoverall printing apparatus generally referred to by the numeral 10, itis common to employ toner particles of finely divided, pigmented or dyedresin, these particles having an average diameter" in the range tomicrons. The carrier particles employed in the developer mix commonlyare from 6 to 8 times larger in diameter than the aforementioned tonerparticles. It has been found that best printing results when thepercentage by weight of toner particles in the overall developer mix isin the range from about 2 percent through 4 percent. Within this range,variations in the percentage of toner have relatively little effect onthe quality of the produced image or coating. 1

Where production rate printing or coating is to be effected, as forexample, in the case where a large number of serially conveyed articlesare to have toner applied thereto, a number of printing operations maybe effected without the addition of toner, provided the percentage oftoner in the developer mix remains within the range indicatedhereinabove. Thorough mixing of the carrier and toner, as by thecrossflow-mixing arrangement 28 discussed hereinabove and by thediversion and reapplication of developer mix discussed hereinabove withrespect to the magnetic drum 33, allows a fuller employment of thedeveloper mix within a printing apparatus prior to the addition oftoner. However, where printing or coating is to be effected atproduction rates, it is desirable to automatically supply additionaltoner, maintaining the percentage toner within the preferred 2 percentthrough 4 percent range, without attention to the number of printingoperations effected by the printing apparatus. Further, it should benoted that where a solid coating is to be applied to a substrate, thenumber of successful printing operations possible between ad ditions oftoner is substantially reduced.

Where the toner and carrier particles constituting the developer mix areof differing optical qualities, that is. differing in reflectivequalities or color, the optical density of the developer will bedependent upon the percentage toner or ratio of toner particles tocarrier particles within the developer mix. Reflection of light from aportion of the developer mix is dependent upon the optical density ofthe mix and as the optical density increases through the increase ofparticles of greater light-absorbent characteristic, the reflection oflight from the portion of developer decreases.

FIG. 2 represents the variation of optical density with variation ofpercentage of toner in a developer mix employing nickel carrierparticles, medium grey in appearance, and X- erox 914 black toner.Employing a Macbeth RD-lOO Densitometer, the optical density indicatedfor the carrier particles alone was slightly less than 1.00 as indicatedat point 45 in FIG. 2. As the percentage toner within the carrier mixwas increased, a substantially linear increase in optical density wasindicated as illustrated by the line 46 in FIG. 2. The relativelysubstantial change in optical density indicated by FIG. 2 where blacktoner particles are employed provides sufficient alteration in the lightreflected from a portion of the mix to allow monitoring of the ratio ofconstituent mix components.

It will be apparent that where the constituent mix com ponent exhibitone or more colors other than white, black and intermediate grey shades,variations in the reflection of white" light from the developer mix willbe substantially reduced for variations in percentage toner. Opticalfiltration of the light applied to the mix, however, may be employed tolimit the spectrum of the light from the illuminating light source tocause reflection from the developer mix to again vary substantially withvariations in the ratio of constituent mix components. Illumination ofthe mix with a colored light beam of a color which is eithersubstantially reflected or sub stantially absorbed by one of the coloredmix components causes the reflective characteristic of the mix toapproximate the reflective characteristics of a system wherein the mixvaries in shades between black and white.

Illustrated in FIG. 3 is the optical density of developer mix employingXerox 22I46" red toner particles and the aforementioned medium greynickel carrier particles, plotted against variations in the percentageof toner particles within the mix. The illustrated measurements whereagain made with a Macbeth RD-IOO Densitometer" and illumination of themix via a red filter indicated an optical density of approximately [.00in the absence of toner as indicated at point 47 in FIG. 3. An increasein the percentage of red toner particles within the mix resulted in agreater reflection of light therefrom and a resultant decrease in theindicated optical density as illustrated by the line 48 of FIG. 3.Similarly, the

employment of a blue filter resulted in a greater absorbing of the lightas the percentage of red toner was increased as indicated by the line 50in FIG. 3.

In FIG. 4 there is illustrated variations in optical density withvariations in the percentage of blue toner of the type designated Xerox22l42" in a mix employing the aforementioned medium grey nickel carrierparticles. The aforementioned densitometer was employed to indicate theoptical densities illustrated in FIG. 4 and a blue filter, limiting thespectrum of the light illuminating the mix to a portion of the bluerange, resulted in a slight decrease in the indicated optical densitywith increases in the percentage of blue toner, as illustrated by theline 51 of FIG. 4, and resultant from a greater reflection of the bluelight with increases in the blueness of the developer mix. However, theblue filter employed did not provide truly significant variations in theindicated optical density, thus illustrating that a blue filter ofslightly differing color characteristic would be preferable if thereflected light was to be employed to indicate the ratio componentparticles in the mix. However, the employment of a red filter provided asubstantial variation in indicated optical density from 1.00,representing a complete deficiency of blue toner as indicated at point50 in FIG. 4 and a substantial increase in optical density withincreases in the percentage of employed blue toner as indicated by theline 52in FIG. 4.

FIG. 6 illustrates the employment of an optical-monitoring system asillustrated in FIG. 1 with suitably selected control provisions forcontrolling the addition of toner to maintain the toner within thepredetermined optimum percentage range. Again, the window 35 is providedhaving a surface 36 coplanar with the interior surface 23 of the housing11. In view of the fact that the developer mix moves directly across thesurface 36 of the window 35 in contact therewith, fabrication of thewindow 35 from sapphire or other abrasion resistant material ispreferable. The light source 37, illuminating the mix through the window35 may be chosen from suitable or commercially available illuminatorssuch as the 1087 Microscope Illuminator available from the AmericanOptical Company. Preferably the photosensitive detector or sensor 40constitutes a photovoltaic cell, again suitably selected or commerciallyavailable, such as the 82M Selenium Photovoltaic Cell" available fromthe International Rectifier Corp. The cell 40 provides a small DC outputsignal representative of the light reflected thereto from the mixadjacent the window 35 and the output signal from the cell 40 is appliedto the input 55 of a suitably selected or commercially available DCamplifier 56. The amplified DC signal is taken from an output 57 of theamplifier 56 and applied to a meter relay 58. The meter relay 58 may beof a known or commercially available type such as the 503L -1 ma. meterand MOdel 901A relay control module combination available from AssemblyProducts, Incorporated and includes meter provisions for indicating themagnitude of the DC current applied thereto. The meter relay 58 providesand output signal therefrom as by the closure of relay contacts includedtherein upon the occurrence of an input of predetermined magnitude, thuscontrolling operation of a suitably selected motor 60 connected indriving relation, as at 61, with the brush 27 of the toner-replenishingprovisions of the overall printing apparatus 10. As the percentage ofthe toner within the developer mix drops to a predetermined percentagewithin the optimum percentage range, the motor 60 is energized effectingreplenishment of toner into the developer mix via the screened window26.

FIG. diagrammatically illustrates the overall operation of the printingapparatus of FIG. I, employing the mix monitoring and control provisionsof FIG. 6. Developer mix is moved along a predetermined path 63 and atpoint 64 a portion of the mix moving along the path 63 is diverted alonga recirculation path 65, while a remaining portion of the mix isdirected, via a path 66 to the printing station or zone 17 forapplication in a printing or coating process. Toner-deficient mix istaken from the printing zone 17 along a path 67 and rejoined with thediverted or recirculated portion of mix at the point 68. The

diverted or recirculated portion of mix, then, has bypassed the printingzone 17 via the path 65 and the percentage toner so diverted has notbeen altered. The combined toner-deficient mix quantity and divertedquantity passes a toner adding station 70 which may include thereservoir, brush and screened window combination illustrated in FIG. 1.Replenishment of toner occurs at the toner-adding station '70 and thereplenished developer mix is passed along a path 71 to acrossflow-mixing station 72 such as the crossflow-mixing rovisions 28illustrated in FIG. 1. The now-substantially homogeneous mix is passedto an optical-monitoring station 73 via a path 74 and at the station 73which may include the monitoring provisions discussed hereinabove, themix is optically monitored and control provisions 74, connected infeedback relation to the toner adding provisions 70 control the additionof toner when the necessity of such an addition is detected at themonitoring station 73. It will be noted from FIG. 5 that the homogeneityof the employed developer mix is greatly increased by the provision ofcrossflow-mixing intermediate the toner-adding station 70 and theoptical-monitoring station 73 and by the recirculation of a divertedportion of the mix of preferred toner/carrier proportions. This providesconsistency in the printing operation occurring at the printing zone 17and assures correct operation of the optical-monitoring provisions byassuring monitoring of a substantially homogeneous portion of developerat the station 73.

While preferred methods and apparatus have been indicated herein,variations from the described preferred em bodiments within the spiritand scope of this invention will be apparent to those skilled in theart.

We claim:

1. An electrostatic printing or coating device comprising a depositoryfor a developer mix of carrier particles and toner particles ofdiffering optical qualities, means for transporting a tonersufficientportion of said developer mix from said depository into an applicationarea, means for applying toner particles from said toner-sufficientportion of said mix in said application area to a substrate causing thatportion of the mix to become a toner-deficient portion, means for movingsaid toner-deficient portion of said mix from said application area to atoner-replenishing station, toner-replenishing means for adding tonerparticles directly to said toner-deficient portion of said mix tocompensate for the toner particles applied to said substrate, viewingwindow means adjacent said tonerreplenishing means along the path ofmovement of said tonerdeficient portion of said mix, optical-sensingmeans adjacent said window means on the opposite side thereof from thepath of movement of said mix, said optical-sensing means including alight source for projecting light through said window against said mix,and a photoelectric sensor for receiving light reflected from said mixthrough sad window and for indicating the ratio of component particlesin said mix, and means electrically connected with said photoelectricsensor for controlling said toner-replenishing means.

2. A device according to claim I wherein said optical sensing meansfurther comprises colored optical filter means, limiting the spectrum oflight projected against said mix by said light source for increasingvariations in light reflected from said mix resultant from variations inthe ratio of component particles in said mix.

3. A device according to claim I wherein said printing or coating deviceincludes an inclined surface lying along the path of movement of saidmix within said device and along which a portion of said mix slides intransit therepast, said window means comprising a transparent panemounted in said inclined surface for sliding and supporting engagementwith said mix to provide self-cleansing of said window.

4. A device according to claim 1 wherein said means electricallyconnected with said photoelectric sensor for controlling saidtoner-replenishing means includes meter-relay means and means connectingsaid photoelectric sensor and said meterrelay means for applying anelectrical signal to said meterrelay means, said meter-relay meanshaving a meter responsive to signals applied thereto for indicating theratio of component particles in said mix and means responsive to apredetermined signal strength for providing an output from saidmeter-relay means.

5. A device according to claim 4 wherein said tonerreplenishing meansincludes a toner reservoir, a screened aperture communicating betweensaid reservoir and the interior of said printing or coating device,rotatably mounted brush means for transporting toner particles to saidscreened aperture through said screened aperture and motor meanselectrically connected with said meter-relay means for actuation by theoutput therefrom.

6. A device according to claim 3 wherein said pane is of sapphire,thereby resisting abrasion of said pane by sliding movement of said mixthereon.

7. A device according to claim l wherein said tonerreplenishing means islocated upstream with respect to said window means and along said pathof movement of said mix, means for mixing the replenished developer mixcomprising crossflow-mixing means intermediate said toner-replenishingmeans and said window means, said crossflow-mixing means including aplurality of mix-contacting obstacles extending into the path of mixmovement.

8. A device according to claim 3 wherein said tonerreplenishing means islocated upstream with respect to said window means along said path ofmovement of said mix, said toner-replenishing means including anaperture opening through said inclined surface for passage of tonertherethrough, means for mixing the replenished developer mix comprisingcrossflow-mixing means intermediate said tonerreplenishing means andsaid window means, said crossflowmixing means including a plurality ofobstacles extending outwardly from said inclined surface for contactingmix in transit on said inclined surface.

9. An electrostatic printing or coating device employing a developer mixof carrier and toner particles comprising means for transporting atoner-sufficient portion of said developer mix into an application area,means for applying toner partication area and for adding said divertedmix portion to said toner-deficient portion in transit from saidapplication area to provide mixing in the direction of developer flow.

10. A device according to claim 9 further comprising crossflow-mixingmeans located in the path of movement of said mix for providing movementof the component particles of the mix at least partially transverse tothe path of movement of the mix.

11. A device according to claim 9 wherein said means for transporting aportion of said mix into an application area comprises a first rotatabledrum for carrying developer mix on the surface thereof, said meansdiverting a portion of the mix comprising a second rotatable drumadjacent said first drum for receiving the diverted portion of mix fromthe surface of said first drum onto the surface of said second drum tobypass said application area.

12. An electrostatic printing or coating device employing a developermix of carrier and toner particles comprising means for transporting aportion of said developer mix along a first path into an applicationarea, means for applying toner particles from said mix to a substrate,means for returning mix along a second path from said application area,tonerreplenishing means located along said second path for replenishingtoner in said mix to compensate for the toner particles applied to saidsubstrate, said toner-replenishing means including means for diverting aportion of said mix in transit on said first path to a point in saidsecond path to provide developer mixing transverse to the direction ofmovement crossflow-mixing means located along said second path followingsaid means for replenishing for providing developer mixing transverse tothe direction of movement of mix, and mix-monitoring means located alongsaid second path follow ing said crossflow-mixing means for detectingthe ratio of component particles in said developer mixv 13. A deviceaccording to claim 12 wherein said means for transporting a portion ofsaid developer mix along a first path includes a first rotatable drumfor carrying developer mix thereon, said means for diverting a portionof the mix comprising a second rotatably drum adjacent said first drumfor l f o a portion f id i i id li i area to a receiving the divertedportion ofmix from said first drum onto said second drum to bypass saidapplication area.

14. A device according to claim 12 wherein said monitoring meanscomprises a window adjacent the flow of mix from said crossflow-mixingmeans and photoelectric sensor means for detecting varying opticalqualities of said mix adjacent said window.

1. An electrostatic printing or coating device comprising a depositoryfor a developer mix of carrier particles and toner particles ofdiffering optical qualities, means for transporting a toner-sufficientportion of said developer mix from said depository into an applicationarea, means for applying toner particles from said toner-sufficientportion of said mix in said application area to a substrate causing thatportion of the mix to become a toner-deficient portion, means for movingsaid tonerdeficient portion of said mix from said application area to atoner-replenishing station, toner-replenishing means for adding tonerparticles directly to said toner-deficient portion of said mix tocompensate for the toner particles applied to said substrate, viewingwindow means adjacent said toner-replenishing means along the path ofmovement of said toner-deficient portion of said mix, optical-sensingmeans adjacent said window means on the opposite side thereof from thepath of movement of said mix, said optical-sensing means including alight source for projecting light through said window against said mix,and a photoelectric sensor for receiving light reflected from said mixthrough sad window and for indicating the ratio of component particlesin said mix, and means electrically connected with said photoelectricsensor for controlling said toner-replenishing means.
 2. A deviceaccording to claim 1 wherein said optical-sensing means furthercomprises colored optical filter means, limiting the spectrum of lightprojected against said mix by said light source for increasingvariations in light reflected from said mix resultant from variations inthe ratio of component particles in said mix.
 3. A device according toclaim 1 wherein said printing or coating device includes an inclinedsurface lying along the path of movement of said mix within said deviceand along which a portion of said mix slides in transit therepast, saidwindow means comprising a transparent pane mounted in said inclinedsurface for sliding and supporting engagement with said mix to provideself-cleansing of said window.
 4. A device according to claim 1 whereinsaid means electrically connected with said photoelectric sensor forcontrolling said toner-replenishing means includes meter-relay means andmeans connecting said photoelectric sensor and said meter-relay meansfor applying an electrical signal to said meter-relay means, saidmeter-relay means having a meter responsive to signals applied theretofor indicating the ratio of component particles in said mix and meansresponsive to a predetermined signal strength for providing an outputfrom said meter-relay means.
 5. A device according to claim 4 whereinsaid toner-replenishing means includes a toner reservoir, a screenedaperture communicating between said reservoir and the interior of saidprinting or coating device, rotatably mounted brush means fortransporting toner particles to said screened aperture through saidscreened aperture and motor means electrically connected with saidmeter-relay means for actuation by the output therefrom.
 6. A deviceaccording to claim 3 wherein said pane is of sapphire, thereby resistingabrasion of said pane by sliding movement of said mix thereon.
 7. Adevice according to claim 1 wherein said toner-replenishing means islocated upstream with respect to said window means and along said pathof movement of said mix, means for mixing the replenished developer mixcomprising crossflow-mixing means intermediate said toner-replenishingmeans and said window means, said crossflow-mixing means including aplurality of mix-contacting obstacles extending into the path of mixmovement.
 8. A device according to claim 3 wherein saidtoner-replenishing means is located upstream with respect to said windowmeans along said path of movement of said mix, said toner-replenishingmeans including an aperture opening through said inclined surface forpassage of toner therethrough, means for mixing the replenisheddeveloper mix comprising crossflow-mixing means intermediate saidtoner-replenishing means and said window means, said crossflow-mixingmeans including a plurality of obstacles extending outwardly from saidinclined surface for contacting mix in transit on said inclined surface.9. An electrostatic printing or coating device employing a developer mixof carrier and toner particles comprising means for transporting atoner-sufficient portion of said developer mix into an application area,means for applying toner particles from a portion of said mix in saidapplication area to a substrate causing that portion of the mix tobecome a toner-deficient portion, means for moving said toner-deficientportion from said application area to a replenishing station,toner-replenishing means for adding toner directly to saidtoner-deficient portion to compensate for the toner particles applied tosaid substrate, said toner-replenishing means including means fordiverting a portion of the mix in transit to said application area andfor adding said diverted mix portion to said toner-deficient portion intransit from said application area to provide mixing in the direction ofdeveloper flow.
 10. A device according to claim 9 further comprisingcrossflow-mixing means located in the path of movement of said mix forproviding movement of the component particles of the mix at leastpartially transverse to the path of movement of the mix.
 11. A deviceaccording to claim 9 wherein said means for transporting a portion ofsaid mix into an application area comprises a first rotatable drum forcarrying developer mix on the surface thereof, said means diverting aportion of the mix comprising a second rotatable drum adjacent saidfirst drum for receiving the diverted portion of mix from the surface ofsaid first drum onto the surface of said second drum to bypass saidapplication area.
 12. An electrostatic printing or coating deviceemploying a developer mix of carrier and toner particles comprisingmeans for transporting a portion of said developer mix along a firstpath into an application area, means for applying toner particles fromsaid mix to a substrate, means for returning mix along a second pathfrom said application area, toner-replenishing means located along saidsecond path for replenishing toner in said mix to compensate for thetoner particles applied to said substrate, said toner-replenishing meansincluding means for diverting a portion of said mix in transit on saidfirst path to a point in said second path to provide developer mixingtransvErse to the direction of movement crossflow-mixing means locatedalong said second path following said means for replenishing forproviding developer mixing transverse to the direction of movement ofmix, and mix-monitoring means located along said second path followingsaid crossflow-mixing means for detecting the ratio of componentparticles in said developer mix.
 13. A device according to claim 12wherein said means for transporting a portion of said developer mixalong a first path includes a first rotatable drum for carryingdeveloper mix thereon, said means for diverting a portion of the mixcomprising a second rotatably drum adjacent said first drum forreceiving the diverted portion of mix from said first drum onto saidsecond drum to bypass said application area.
 14. A device according toclaim 12 wherein said monitoring means comprises a window adjacent theflow of mix from said crossflow-mixing means and photoelectric sensormeans for detecting varying optical qualities of said mix adjacent saidwindow.